1. Field of the Invention
This invention relates to a fuel metering control system for an internal combustion engine.
2. Description of the Prior Art
The PID control law is ordinarily used for fuel metering control for internal combustion engines. The control error between the desired value and the manipulated variable (control input) is multiplied by a P term (proportional term), an I term (integral term) and a D term (differential or derivative term) to obtain the feedback correction coefficient (feedback gain). In addition, it has recently been proposed to obtain the feedback correction coefficient by use of modern control theory or the like. As the control response is relatively high in such cases, however, it may under some engine operating conditions become necessary to use a lower control response in order to prevent the control from becoming unstable owing to controlled variable fluctuation or oscillation.
It has therefore been proposed, as in Japanese Laid-Open Patent Application No. Hei 4(1992)-209,940, to calculate a first feedback correction coefficient using modern control theory, calculate a second feedback correction coefficient whose control response is inferior to (or lesser than) that of the first feedback correction coefficient using the PI control law, and determine the controlled variable using the second feedback correction coefficient during engine deceleration, when combustion is unstable. For a similar reason, Japanese Laid-Open Patent Application No. Hei 5(1993)-52,140 proposes determining the controlled variable using a second feedback correction coefficient of inferior control response when the air/fuel ratio sensor is in the semi-activated state. In U.S. patent application No. 08/401,430 filed on Mar. 9, 1995, for example, the assignee proposes a system for determining the quantity of fuel injection using an adaptive controller.
In fuel metering control, the supply of fuel is shut off during cruising and certain other operating conditions and, as shown in FIG. 17, it is controlled in an open-loop (O/L) fashion during the fuel cutoff period. Then when the fuel supply is resumed for obtaining a stoichiometric air/fuel ratio (14.7:1), for example, fuel is supplied based on the quantity of fuel injection determined in accordance with an empirically obtained characteristic. As a result, the true air/fuel ratio (A/F) jumps from the lean side to 14.7. However, a certain amount of time is required for the supplied fuel to be combusted and for the combusted gas to reach the air/fuel ratio sensor. In addition, the air/fuel ratio sensor has a detection delay time. Because of this, the detected air/fuel ratio is not always the same as the true air/fuel ratio but, as shown by the broken line in FIG. 17, involves a relatively large error.
At this time, as soon as the high-control-response feedback correction coefficient (illustrated as KSTR in the figure) is determined based on a control law such as the adaptive control law proposed by the assignee, the adaptive controller determines the feedback correction coefficient KSTR so as to immediately eliminate the error between the desired value and the detected value. As this difference is caused by the sensor detection delay and the like, however, the detected value does not indicate the true air/fuel ratio. Since the adaptive controller nevertheless absorbs the relatively large difference all at one time, KSTR fluctuates widely as shown in FIG. 17, thereby also causing the controlled variable to fluctuate or oscillate and degrading the control stability.
It is therefore preferable to determine one feedback correction coefficient of high control response using a control law such as the adaptive control law and another feedback correction coefficient of low control response using a control law such as the PID control law (illustrated as KLAF in the figure) and to select one or the other of the feedback correction coefficients depending on the engine operating condition. Since the different types of control laws have different characteristics, however, a sharp difference in level may arise between the two correction coefficients. Because of this, switching between the correction coefficients is liable to destabilize the controlled variable and degrade the control stability. None of the aforesaid prior art references offer any measure for overcoming this problem.
Moreover, when the engine is equipped with a variable valve timing mechanism which switches the opening/closing timing of the intake and/or exhaust valves between two types of characteristics, i.e., the characteristic for low-engine-speed and that for high-engine-speed, a large amount of valve timing overlap present when the high engine speed characteristic is selected is apt to cause intake air blowby (escape of intake air through the exhaust valve). At that situation, the detected exhaust ratio is not likely to be stable. The condition may therefore become different from that of the high-control-response feedback correction coefficient such as that determined by an adaptive control law, which may occasionally make it impossible to continue the feedback control using the high-control-response feedback correction coefficient.
An object of the invention is therefore to provide a fuel metering control system for an internal combustion engine which determines feedback correction coefficients different in control response using multiple types of control laws and which smooths the switching between the feedback correction coefficients, while ensuring the continuities of feedback control even when the engine is equipped with a variable valve timing mechanism that switches the opening/closing timing of the intake and/or exhaust valves between a plurality of characteristics.